Head drive control device and inkjet recording device

ABSTRACT

In an electrostatic head, each of electrostatic actuators includes a diaphragm doubling as or including a first electrode forming a wall surface of a discharge room communicating with a nozzle discharging a drop, and a second electrode opposing the first electrode. The diaphragm is deformed by generating an electrostatic force between the first electrode and the second electrode. The first electrodes or the second electrodes of the electrostatic actuators are combined electrically. Differently polarized potentials are applied to the first electrode and the second electrode upon discharging the drop.

TECHNICAL FIELD

The present invention generally relates to a head drive control deviceand an inkjet recording device, and more particularly, to a head drivecontrol device included in an inkjet recording device which records animage by discharging ink drops.

BACKGROUND ART

As an inkjet head forming a recording head of an inkjet recording deviceused as an image recording device or an image forming device, such as aprinter, a facsimile, and a copying machine, a head using anelectrostatic actuator as disclosed in Japanese Laid-Open PatentApplication No. 2001-260346 is well known.

This electrostatic inkjet head includes electrostatic actuators in eachof which a diaphragm used also as or including a first electrode forminga wall surface of a discharge room communicating with a nozzle and asecond electrode (an individual electrode) are opposed to each otherwith a predetermined air gap therebetween. A driving waveform is appliedbetween the first electrode and the second electrode of thiselectrostatic actuator so as to deform the diaphragm of each actuator byutilizing an electrostatic attraction. By a mechanical force upon thedeformation, or by a mechanical restitution produced in the diaphragmupon turning off the electrostatic attraction, an ink in the dischargeroom is discharged from the nozzle.

In a drive control device for a head using such electrostatic actuators,first electrodes of the electrostatic actuators are combinedelectrically to form a common electrode, and the first electrodesforming the common electrode are set to 0V, and upon discharging an inkdrop, a pulse-form potential of +V is selectively applied to individualelectrodes (second electrodes).

Besides, as a drive control device for driving an electrostaticactuator, PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998 “AMEMS-Based Projection Display” describes an example in which a nonzeropotential is applied to both electrodes composing an actuator of anoptical mirror. This drive control device applies a bias potential to areflector plate, and applies an address potential to an electrodedetermining a direction of the reflector plate. Upon each control, apotential of 24V to −26V is applied to the reflector plate, and apotential of 0V or 5V is applied to the address electrode. This mannerof applying the voltages is devised for maximizing a function of theoptical mirror, thereby enabling the reflector plate to surely swing at+10 degrees or −10 degrees according to a control signal, with aremarkably high reliability.

By the way, an inkjet recording device, such as an inkjet printer, isrequired to have a high total performance, such as an output speed (arecording speed) and an image quality. To fulfill these requirements, adegree of nozzle concentration at a head is raised so as to increase anumber of nozzles.

At this point, in view of a relation between an improvement of thedegree of nozzle concentration and a head structure, generally, unlike athermal head discharging an ink from a nozzle by using a pressure ofcavities generated by causing the ink to undergo a film boiling by usinga heating resistor, a piezoelectric or electrostatic head which includesa diaphragm having a low rigidity, and discharges an ink by varying thisdiaphragm, has a difficulty in raising the degree of concentration.

In order to raise the degree of concentration in an electrostatic head,a shorter-side width (a width in a direction in which nozzles arearranged) of a diaphragm has to be shortened, whereas a volume ofdischarged an ink drop has to be secured to a certain degree. Therefore,in order to shorten the shorter-side width of the diaphragm, adisplacement of the diaphragm needs to be enlarged. In this case, from asimple viewpoint, thinning a thickness of the diaphragm can enlarge thedisplacement even though the shorter-side width is short; however, froma viewpoint of discharging a drop, the diaphragm needs to have a certaindegree of rigidity, thereby limiting a range in which the diaphragm canbe thinned.

That is, an electrostatic attraction generated in an electrostaticactuator can be represented by the following expression (1), where V isa driving voltage, g is a gap length (a distance between an individualelectrode and a common electrode), and δ is a displacement of adiaphragm.F=(ε0/2)·V ²/(g−δ)²  <Expression 1>

As mentioned above, when the degree of nozzle concentration is raised,the gap length g should be increased. According to the expression (1),when the gap length g becomes large, the driving voltage V also needs tobe raised in order to obtain the electrostatic attraction of the samemagnitude. Further, as the gap length g becomes larger, the diaphragmdisplacement δ has a smaller variation range in relation to a variationrange of the driving voltage V; therefore, even an slight enlargement ofthe gap length g calls for a large increase in the driving voltage V. Inother words, when the degree of concentration is raised whilemaintaining a capability of discharging drops, the driving voltage of anactuator tends to be made higher.

Such an increase in the driving voltage means not only an increase inpower consumption but also an increase in a withstand pressure of atransistor composing a drive control device (a driver) controlling theactuator. In general, as a size of a transistor becomes larger, awithstand pressure of the transistor becomes higher, although dependingalso on a thickness of an oxide film of the transistor. Besides, as thewithstand pressure becomes higher, a manufacturing process also becomesmore costly. As a result, the increase in the driving voltage leads tothe cost of the drive control device becoming higher. In this case,since an inkjet head includes many actuators, the increase in the costof the head drive control device becomes large.

Besides, an actuator in a drop discharge head needs to have a functionof bending a diaphragm toward electrodes by turning a voltage on betweenthe electrodes, and a function of returning the diaphragm to theoriginal position by turning the voltage off. Therefore, from afunctional viewpoint, there is no need for using a bias method as usedin the driving method described in PROCEEDINGS OF THE IEEE, VOL. 86, NO.8, AUGUST 1998 “A MEMS-Based Projection Display” as above; instead,applying a required potential to one electrode and setting anotherelectrode to GND may be sufficient. Yet, when using the bias method, thevoltage does not need to be changed positive and negative upon eachcontrol (for discharging one drop); rather, this impairs functions ofthe head. Besides, it is not necessary to apply an especially largepotential to one of electrodes. Therefore, the driving method describedin PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998 “A MEMS-BasedProjection Display” cannot be simply applied to a drive control devicefor a head.

DISCLOSURE OF INVENTION

It is a general object of the present invention to provide an improvedand useful head drive control device and an inkjet recording device, inwhich the above-mentioned problems are eliminated.

A more specific object of the present invention is to provide a headdrive control device capable of driving a drop discharge head having ahigh degree of nozzle concentration at a low cost, and an inkjetrecording device including the head drive control device.

In order to achieve the above-mentioned objects, there is providedaccording to one aspect of the present invention a head drive controldevice for driving a head in which first electrodes or second electrodesof a plurality of electrostatic actuators are combined electrically, thehead drive control device including a part applying differentlypolarized potentials to the first electrode and the second electrodeupon discharging a drop.

According to the present invention, the head drive control device candrive an inkjet head having a high degree of nozzle concentration at alow cost.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded perspective view of an example of an inkjet headdriven and controlled by a head drive control device according to thepresent invention;

FIG. 2 is a plan view of transparently showing a nozzle board of thehead shown in FIG. 1;

FIG. 3 is an illustrative sectional view of the head, taken along alonger-side direction of a diaphragm;

FIG. 4 is an illustrative sectional view of the head, taken along ashorter-side direction of the diaphragm;

FIG. 5 is a graph representing a relation between a bending amount of afirst electrode (the diaphragm) and a driving voltage in anelectrostatic actuator;

FIG. 6A to FIG. 6E are waveform diagrams showing various examples ofdriving waveforms applied by the head drive control device according tothe present invention to a common electrode and individual electrodes ofthe electrostatic actuator;

FIG. 6F is a waveform diagram showing conventional driving waveformsapplied to a common electrode and individual electrodes of anelectrostatic actuator;

FIG. 7 is a sectional view of main parts of another example of a headpreventing a residual electric charge;

FIG. 8 is a sectional view of main parts of still another example of ahead preventing a residual electric charge;

FIG. 9 is a block diagram of a structure of the head drive controldevice according to the present invention;

FIG. 10 is a diagram showing a relation between driver modules andactuators in the head drive control device shown in FIG. 9;

FIG. 11 is a circuit diagram of a basic circuit structure of one levelshifter in the head drive control device shown in FIG. 9;

FIG. 12 is a circuit diagram of a basic circuit structure of anotherlevel shifter in the head drive control device shown in FIG. 9;

FIG. 13 is a circuit diagram of a basic circuit structure of an analogswitch in the head drive control device shown in FIG. 9;

FIG. 14 is a perspective view of a mechanism part of an inkjet recordingdevice according to the present invention; and

FIG. 15 is a side sectional view of the recording device.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will now be given, with reference to the drawings, ofembodiments according to the present invention.

First, a description will be given, with reference to FIG. 1 to FIG. 5,of an example of an inkjet head as a drop discharge head driven by ahead drive control device according to the present invention. FIG. 1 isan exploded perspective view of the head. FIG. 2 is a plan view oftransparently showing a nozzle board of the head. FIG. 3 is anillustrative sectional view of the head, taken along a longer-sidedirection of a diaphragm. FIG. 4 is an illustrative sectional view ofthe head, taken along a shorter-side direction of the diaphragm.

This inkjet head has a laminated structure in which a channel substrate1 as a first substrate, an electrode substrate 3 as a second substrateprovided under the channel substrate 1, and a nozzle board 4 as a thirdsubstrate provided over the channel substrate 1 are joined one overanother, thereby forming discharge rooms 6 communicating with aplurality of nozzles 5, and a common liquid room 8 communicating withthe discharge rooms 6 via a fluid resistance part 7.

In the channel substrate 1 are formed the discharge rooms 6, diaphragms10 forming wall surfaces serving as bottom parts of the discharge rooms6, protruding parts forming partitions 11 separating the discharge rooms6, a receding part forming the common liquid room 8, and so forth. Thecommon liquid room 8 is formed so that a capacity thereof becomes 20times as much as or less than a capacity of each of the discharge rooms6.

In the channel substrate 1, the diaphragms 10 having a desired thicknessare formed as follows: a boron which is a highly concentrated impurityis diffused on a single-crystal silicon substrate (silicon wafer) of a(110) plane direction into a thickness (depth) corresponding to thediaphragms; this highly concentrated boron doped layer is used as anetching stop layer in performing an anisotropic etching to form recedingparts becoming the discharge rooms 6 and so forth, thereby leaving thediaphragms 10 having the desired thickness. Besides, other than theboron, a gallium, an aluminum and so forth can be used as the highlyconcentrated P-type impurity.

Besides, the diaphragms 10 may be formed by a method of forming anN-type layer becoming the diaphragms on a P-type substrate, or forming aP-type layer becoming the diaphragms on an N-type substrate, andstopping an etching according to an electrochemical etching, by a methodof using an SOI substrate and stopping an etching by an oxide filmlayer, or by a method of controlling a time to terminate an etching.

In the electrode substrate 3, receding parts 14 are formed, andelectrodes 15 opposing the diaphragms 10 with a predetermined gap 16therebetween are formed on respective bottom surfaces of the recedingparts 14. The electrodes 15 and the diaphragms 10 compose actuator partsdeforming the diaphragms 10 by an electrostatic force to vary theinternal capacity of the discharge rooms 6. Joining the electrodesubstrate 3 to the channel substrate 1 forms the gap 16, and disposesthe electrodes 15 at respective positions corresponding to thediaphragms 10.

For the purpose of preventing the electrodes 15 of the electrodesubstrate 3 from being damaged by contacting the diaphragms 10, aninsulating layer 17, such as SiO₂ in 0.1 μm thickness, is formed on eachof the electrodes 15. In addition, an electrode pad part 15 a used forconnecting to an external drive circuit via a connection part is formedby extending the electrode 15 to near an end of the electrode substrate3.

Besides, an opposing contact part 18 contacting the diaphragm 10 beingdeformed is formed between the electrodes 15 in a substantially centralpart in the shorter-side direction of the diaphragm. The opposingcontact part 18 is formed on the bottom surface of the receding part 14in a same process with the electrodes 15. The insulating layer 17 isformed also on a surface of the opposing contact part 18. The opposingcontact part 18 and the diaphragm 10 are electrically connected so as toassume an identical potential when the opposing contact part 18 and thediaphragm 10 contact each other. This prevents an occurrence of aresidual electric charge upon the contact of the diaphragm 10, asdescribed in Japanese Laid-Open Patent Application No. 2001-260346.

Besides, in the electrode substrate 3 is formed an ink supply opening 9which is a through hole used for supplying an ink to the common liquidroom 8 from outside. A through hole 9 a is formed in the common liquidroom 8 of the channel substrate 1 at a part corresponding to the inksupply opening 9.

The electrode substrate 3 is formed as follows: the receding parts 14are formed by etching using an HF solution, etc., in a glass substrateor a single-crystal silicon substrate including a thermally-oxidizedfilm 3 a formed on a surface thereof; an electrode material, such as atitanium nitride, having a high heat resistance is formed into a filmhaving a desired thickness in the receding part 14 by a film-formingtechnique, such as a sputtering, a CVD, or a deposition; thereafter, theelectrodes 15 are formed only in the receding part 14 by forming aphotoresist and etching the film. The electrode substrate 3 and thechannel substrate 1 are joined to each other by a process, such as ananodic junction or a direct junction.

In the present embodiment, the electrodes 15 and the opposing contactpart 18 are formed by sputtering the titanium nitride into a thicknessof 0.1 μm in the receding part 14 having a depth of 0.4 μm formed byetching the silicon substrate; thereon, an SiO₂ sputtering film isformed as the insulating layer 17 in a thickness of 0.1 μm. Accordingly,in the present head, the air gap 16 has a length (an interval betweenthe diaphragm 10 and a surface of the insulating layer 17) of 0.2 μmafter joining the electrode substrate 3 and the channel substrate 1.

In the nozzle board 4, the nozzles 5 and a groove forming the fluidresistance part 7 are formed, and a water-repellent finishing is appliedto a discharge surface. The nozzle board 4 is formed of a resinmaterial, such as a polyimide, and is joined to the channel substrate 1with an adhesive. The nozzle board 4 forms a wall surface of the commonliquid room 8.

In the present head, the diaphragm 10 is connected to a commonelectrode, and the electrode pad 15 a is bonded with a lead, and isconnected with a driver not shown in the figures, thereby enabling theinkjet head to be driven.

Besides, an ink supply pipe may be joined to the ink supply opening 9,thereby enabling the common liquid room 8 and the discharge rooms 6 andso forth to be filled with an ink supplied from an ink tank (not shownin the figures) via the ink supply opening 9. The ink used is preparedby dissolving or diffusing a surface-active agent, such as an ethyleneglycol, and a dye or a pigment, in a main solvent, such as water,alcohol or toluene. Further, attaching a heater to the inkjet headenables a use of a hot-melt ink.

With the above-described structure, when a positive voltage pulse, forexample, is impressed to the electrode 15 by the driver so that asurface of the electrode 15 is charged at a positive potential, anundersurface of the corresponding diaphragm 10 is charged at a negativepotential. Accordingly, the diaphragm 10 is attracted by anelectrostatic force so as to bend toward a direction narrowing aninterval with the individual electrode 15. In this course, the bendingof the diaphragm 10 causes the ink to be supplied from the common liquidroom 8 via the fluid resistance part 7 to the discharge room 6.

Subsequently, when the voltage pulse impressed to the electrode 15 isturned off so as to discharge a stored electric charge, the diaphragm 10is restored to the original position. This restoration action causes aninternal pressure of the discharge room 6 to rise sharply so that an inkdrop is discharged from the nozzle 5 toward a recording sheet (not shownin the figures).

Next, a description will be given of the head drive control deviceaccording to the present invention for driving the head using theabove-described electrostatic actuators.

First, a description will be given of a reason why a driving method ofimpressing differently polarized potentials to first and secondelectrodes is possible as in the head drive control device according tothe present invention. In the above-described inkjet head, thediaphragms forming the first electrodes are unitary throughout theactuators; therefore, the first electrodes of the actuators are combinedelectrically. Generally, electrodes electrically combined among theactuators are referred to as common electrode, electrodes not combinedelectrically among the actuators are referred to as individualelectrodes, and a voltage impressed to the common electrode is referredto as “bias voltage”.

For simplicity's sake, it is assumed that a diaphragm fixed at foursides receives a uniform load by an electrostatic attraction. On thisassumption, a bending amount δ of the diaphragm is represented by thefollowing expression (2), where E is a Young's modulus of a material ofthe diaphragm, h is a thickness of the diaphragm, ν is a Poisson's ratioof the material of the diaphragm, a is a shorter-side width of thediaphragm, and δ is the bending amount of the diaphragm.F=(32·E·h3/(1−ν2)/a ⁴)·δ  <Expression 2>

Ignoring effects of gases existing between both electrodes, andsubstituting specific values shown in Table 1 for the values in theabove-mentioned expressions (1) and (2), a δ-V curve is represented in agraph as shown in FIG. 5.

TABLE 1 Parameters used in calculation Distance between electrodes(excluding 0.3 electrode insulating layers) (μm) Material of electrodeinsulating layers SiO₂ Total thickness of electrode insulating 0.2layers (μm) Young's modulus of diaphragm (GPa) 290 Poisson's ratio ofdiaphragm 0.293 Shorter-side width of diaphragm (μm) 120 Thickness ofdiaphragm (μm) 2

As shown in FIG. 5, although curves A and B are obtained theoretically,the curve B does not stand in actuality; instead, characteristics of acurve C are realized with respect to bending amounts in a domain of thecurve B.

As shown in FIG. 5, a maximum bending amount (a maximum displacement) is0.3 μm, and a maximum driving voltage is 23.6V; even when 21V, which isequivalent to 90% of the maximum driving voltage, is impressed, thebending amount is 0.06 μm equivalent to 20% of the maximum displacement.That is, when a drop is discharged from a nozzle by driving one actuatorwhile impressing a bias voltage of 21V to a common electrode, the biasvoltage of 21V is impressed also to other actuators not dischargingdrops; however, there occurs no trouble, such as drops discharged bythis bias voltage, because the bending amount is only 0.06 μm equivalentto 20% of the maximum displacement.

In reality, because of gases existing between both electrodes, the δ-Vcurve is not represented in the graph as shown in FIG. 5; however, thequalitatively same thing can be applied.

By a conventional head drive control device, a common electrode is setto GND, and upon discharging a drop, a + potential is applied toindividual electrodes, as shown in FIG. 6F.

By contrast, by the head drive control device according to the presentinvention, differently polarized potentials are applied to a commonelectrode and individual electrodes upon discharging a drop, as shown inFIG. 6A to FIG. 6E.

Specifically, in a first example shown in FIG. 6A, a + potential (a +bias voltage) is applied to the common electrode, and a − potential isapplied to the individual electrodes in a pulse waveform upondischarging. In a second example shown in FIG. 6B, a + potential isapplied to the common electrode in a pulse waveform upon discharging,and a − potential is applied to the individual electrodes in a pulsewaveform at substantially the same time. In a third example shown inFIG. 6C, potentials applied to the common electrode and the individualelectrodes have pulse waveforms alternately reversing polarities; upondischarging, a + potential is applied to the common electrode, and a −potential is applied to the individual electrodes; upon nextdischarging, a − potential is applied to the common electrode, and a +potential is applied to the individual electrodes.

In a fourth example shown in FIG. 6D, a + potential is applied to thecommon electrode in a pulse waveform upon discharging, and a − potentialis applied to the individual electrodes in a pulse waveform atsubstantially the same time, wherein the potentials applied to thecommon electrode and the individual electrodes have absolute values ofmaximum values set substantially equal. Similarly, in a fifth exampleshown in FIG. 6E, a − potential is applied to the common electrode in apulse waveform upon discharging, and a + potential is applied to theindividual electrodes in a pulse waveform at substantially the sametime, wherein the potentials applied to the common electrode and theindividual electrodes have absolute values of maximum values setsubstantially equal.

Accordingly, providing the head drive device with a part for generatingthe driving waveforms shown in FIG. 6A to FIG. 6E enables differentlypolarized potentials to be applied to the common electrode and theindividual electrodes upon discharging.

The driving waveforms shown in FIG. 6A and FIG. 6B are adoptable whenthe actuator has a structure in which a residual electric charge doesnot occur, or is removed. Specifically, as in the inkjet head describedabove, it is preferred that the actuator employs the structure in whichthe opposing contact part 18 is provided between the electrodes 15, andthe opposing contact part 18 and the diaphragm 10 assume an identicalpotential upon the diaphragm 10 contacting the opposing contact part 18.

Besides, a structure preventing a residual electric charge is notlimited to the structure employed in the above-mentioned inkjet head,and other structures such as shown in FIG. 7 and FIG. 8 are alsoadoptable. In these structures, a protruding part at which a diaphragmand an electrode can contact is provided on the diaphragm, and thediaphragm and the electrode assume an identical potential at thisprotruding contact part.

Specifically, in the structure shown in FIG. 7, an insulating film 31formed on an electrode-side surface of a diaphragm 30 forms a protrudingpart 32 opposing electrodes. On the other hand, on the electrodesubstrate 3, electrodes 35 and 34 opposing the diaphragm 30 with a gap36 therebetween are provided, and a separate electrode 38 separated fromthe electrodes 35 and 34 is provided. The separate electrode 38 islocated at a position contacting the protruding part 32 when thediaphragm 30 deforms. Besides, an insulating film 37 is formed onsurfaces of the electrodes 35 and 34 and the separate electrode 38. Theseparate electrode 38 and the diaphragm 30 are electrically connected.

According to this structure, upon driving the actuator, a potentialapplied to the diaphragm 30 is forcibly applied to the separateelectrode 38.

In the structure shown in FIG. 8, the insulating film 31 formed on theelectrode-side surface of the diaphragm 30 forms the protruding part 32opposing the electrodes. Besides, a separate electrode 33 separatedelectrically from the diaphragm 30 by the insulating film 31 is formedat a backside of the protruding part 32. On the other hand, on theelectrode substrate 3, the electrodes 35 and 34 opposing the diaphragm30 with the gap 36 therebetween are provided, and the separate electrode38 separated from the electrodes 35 and 34 is provided. The separateelectrode 38 is located at the position contacting the protruding part32 when the diaphragm 30 deforms. Besides, the insulating film 37 isformed on the surfaces of the electrodes 35 and 34 and the separateelectrode 38. The separate electrode 38 and the separate electrode 33are electrically connected.

According to this structure, a potential of a contact part (the separateelectrodes 38 and 33) can be determined regardless of a potentialapplied upon driving the actuator. In this case, the potential of bothelectrodes 38 and 33 can be set to GND constantly.

Besides, in the structures shown in FIG. 7 and FIG. 8, the protrudingpart is formed by the insulating film; however, the protruding part maybe formed by an electrode material.

In addition, it is preferred that the driving waveforms shown in FIG. 6Care adopted when the actuator has a structure in which an occurrence ofa residual electric charge is not prevented. That is, this drivingwaveform applies a pulse potential having a polarity reversed from apreceding pulse to each electrode so as to neutralize the residualelectric charge.

Besides, in the above-described structures, one dot in an image isformed by one drop discharged from the nozzle; however, these structuresare similarly applicable when one dot in an image is formed by severaldrops discharged from the nozzles, i.e., when a dot is formed bydischarging a plurality of ink drops in one driving cycle.

Next, a description will be given, with reference to FIG. 9, of astructure of the head drive control device according to the presentinvention.

The present head drive control device includes a drive control part 51for selectively applying a driving potential to the individualelectrodes 15 of a plurality of the electrostatic actuators, and adriver module 52 for applying a driving potential to the diaphragm 10 asthe common electrode. Besides, at least the drive control part 51 andthe driver module 52 form a part applying differently polarizedpotentials to the first electrode and the second electrode upondischarging a drop.

The drive control part 51 has a structure as follows, as in a generalhead drive control device. In this structure, image data supplied from amain control part not shown in the figures is transmitted to a shiftregister 53 serially in synchronization with a clock, is converted intoparallel data, and is stored in a latch circuit 54 temporarily. Anactuator to be driven is selected by a selector 55. A logic drivingvoltage of 5V is converted into a predetermined voltage capable ofdriving a switch 58 by a level shifter 57 in one of driver modules 56(only one driver module 56 shown in FIG. 9) provided according to anumber of the actuators as shown in FIG. 10, and is supplied to theswitch (analog switch) 58. The driving voltage is supplied to the switch58 so as to turn on the switch 58, thereby applying the driving voltageto the individual electrode 15.

On the other hand, a driving voltage is applied from the driver module52 to the diaphragm 10 which is the common electrode.

The shift register 53, the latch circuit 54 and the selector 55 areso-called logic parts, which are driven by (0V, 5V); therefore, aconstituent transistor composing these parts may only have a withstandpressure of 5V. On the other hand, a withstand pressure of the levelshifter 57 and the switch 58 composing the driver module 56 depends onthe driving voltage of the actuator; when the driving voltage is high,the withstand pressure of the constituent transistor has to be alsohigh. That is, when the driving voltage of the actuator rises, a cost ofthe driver also rises.

Besides, basic circuits of the level shifter 57 composing the drivermodule 56 are shown in FIG. 11 and FIG. 12, and a basic circuit of theswitch 58 is shown in FIG. 13. Besides, the level shifter shown in FIG.11 is a positive voltage conversion type, and the level shifter shown inFIG. 12 is a negative voltage conversion type.

As mentioned above, the withstand pressure of the level shifter 57 andthe switch 58 depends on the driving voltage of the actuator. In theelectrostatic head, the individual electrodes 14 exist according to thenumber of the actuators, and a number of the common electrode is limitedto one or several. Therefore, the drive control device requires manydriver modules 56 for the individual electrodes, and one or severaldriver modules 52 for the common electrode.

A withstand pressure required for a transistor composing each of thedriver modules approximately equals a voltage used upon driving theactuator. Accordingly, when a total voltage impressed to the actuatorsis 80V, for example, with +30V being impressed to the common electrodeand −50V being impressed to the individual electrodes, a withstandpressure of transistors composing the driver module for the commonelectrode is approximately 30V, and a withstand pressure of transistorscomposing the driver modules for the individual electrodes isapproximately 50V.

In actuality, for reasons mentioned hereinafter, the withstand pressureof the transistor used for applying a negatively-polarized potentialrises slightly. In a conventional drive control device, since the commonelectrode is set to GND, the driver module for the common electrode isnot required. According to the drive control device of the presentinvention, even when the driving voltage of the electrostatic actuatorrises, the withstand pressure of the transistors composing the drivermodule can be suppressed. This is more advantageous in terms of coststhan increasing the number of the driver modules for the commonelectrode by one or several.

Further, since only the basic circuit structures are shown in FIG. 11 toFIG. 13, and compensations for changes in power supply voltage andchanges in temperature and so forth are required in reality, thecircuits become more complicated with a larger number of transistors. Inthis case, the decrease in the withstand pressure of the transistors asa result of using the head drive control device according to the presentinvention enables a large cost reduction of the drive control device.

Besides, in the head drive control device according to the presentinvention, upon discharging a drop, the change of the driving voltage ofthe actuator may be controlled by a magnitude of a potential impressedto the individual electrodes while a magnitude of a potential impressedto the common electrode may be fixed; this does not complicate thedriver module for the common electrode, and therefore is morepreferable.

Besides, in the present embodiment, the diaphragms form the commonelectrode; however, a manufacturing process determines which of thediaphragm and the opposing electrodes should form the common electrode;therefore, the present invention is similarly applicable when theopposing electrodes form the common electrode.

Next, a description will be given of maximum values of potentialsimpressed to the first and second electrodes. The potentials impressedto the first and second electrodes have substantially the same absolutevalue as in the fourth and fifth examples shown in FIG. 6D and FIG. 6E.That is, it is preferred that the absolute values of the maximumvoltages are set substantially the same.

Besides, the above-mentioned “maximum voltage” includes a margin voltagefor temperature compensation and so forth. Additionally, “substantiallythe same” means that withstand pressures of P-channel MOSFETs andN-channel MOSFETs composing the driver modules are substantially thesame. Strictly, however, for reasons mentioned hereinafter, it ispreferred that an absolute value of a maximum value of anegatively-polarized potential among the potentials impressed to thefirst and second electrodes is set lower than an absolute value of amaximum value of a positively-polarized potential thereamong byapproximately 5V, for example, which is equivalent to a voltage used forthe logic parts of the drive control device.

Accordingly, it becomes unnecessary to provide a transistor having aprominently large withstand pressure. That is, there is no element whichoccupies a large area by itself. Besides, making the withstand pressuressubstantially the same avoids complicating a manufacturing process, as aresult of which a total cost of the drive control device includingmaterials and manufactures thereof can be reduced.

Especially, as in the inkjet head, when there are hundreds of nozzles,i.e., many actuators, the reduction of the total cost exhibits a largeadvantage. Specifically, in the drive control device controlling a96-bit electrostatic actuator made on an experimental basis, 96 drivermodules using transistors having a large withstand pressure wererequired according to the conventional driving voltage impressing method(FIG. 6F). On the other hand, in the head drive control device (theexample of FIG. 6D) according to the present invention, only 97 (=96+1)driver modules using transistors having half the withstand pressure wererequired. Thus, it was confirmed that a large cost reduction can beachieved for the drive control device as a whole.

Besides, the above-described bias driving method for the commonelectrode is completely different from a driving method used in aconventional optical mirror and so forth in respect of methods forimpressing a voltage and effects thereof.

Next, a description will be given of waveforms of potentials impressedto the first and second electrodes. Potentials impressed to the commonelectrode combined electrically throughout the actuators are preferredto have pulse waveforms as shown in FIG. 6B to FIG. 6E. In this case,pulse voltages impressed to the common electrode and the individualelectrodes are preferred to have substantially the same pulse width.

That is, other than the pulse voltage, a direct-current voltage (FIG.6A) can also be impressed to the common electrode, and impressing thedirect-current voltage or the pulse voltage does not result insubstantially different characteristics. However, biasing with thedirect-current voltage produces a merit of simplifying circuitstructures.

However, in a head having a small sum of a restitution force of thediaphragm upon turning off the voltage, i.e., a restitution force due tothe rigidity of the diaphragm, and a repulsive force of gases existingbetween electrodes after compression, the presence of the direct-currentbias may inhibit the restitution of the diaphragm.

With reference to the theoretical curves A and B shown in FIG. 5ignoring the existence of gases (air) between electrodes, on theabove-mentioned assumption, in order to separate the diaphragm oncecontacting the electrode therefrom, the voltage impressed to theactuator theoretically needs to be set to 0V.

However, in an actual system, when the impressed voltage is reduced to acertain value, the diaphragm separates from the electrode, because: theelectrostatic attraction generated upon the diaphragm contacting theelectrode is not infinite because of the existence of the electrodeinsulating film provided for preventing an electric short circuit uponboth electrodes contacting each other; and a repulsive/expansive forceoccurs after gases existing between electrodes are compressed. Accordingto this mechanism, when the voltage impressed to the diaphragmcontacting the electrode is reduced to a certain value, the diaphragmseparates from the electrode.

Thereupon, by making actuators 1 and 2 having parameters shown in Table2 on an experimental basis, maximum voltages capable of biasing thecommon electrode are investigated. In both actuators, it is assumed thatthe diaphragm contacts the electrode upon discharging a drop.

TABLE 2 Parameters of experimental-basis actuators and maximum voltagescapable of biasing Actuator 1 Actuator 2 Distance between electrodes 0.50.2 (excluding electrode insulating layers) (μm) Material of electrodeinsulating SiO₂ SiO₂ layers Total thickness of electrode 0.2 0.3insulating layers (μm) Material of diaphragm SiN Si Young's modulus ofdiaphragm (GPa) 290 170 Shorter-side width of diaphragm 90 125 (μm)Thickness of diaphragm (μm) 0.9 2 Pulse width of impressed voltage 6 6(μs) Voltage causing diaphragm to 67 37 contact electrode (V) Maximumvoltage capable of biasing 54 23 (V)

As shown in Table 2, the voltage causing the diaphragm to contact theelectrode is 67V for the actuator 1, and is 37V for the actuator 2, andthe maximum voltage capable of biasing is 54V for the actuator 1 and is23V for the actuator 2. In the drop discharge head, the driving voltageneeds to have a certain range so as to compensate for changes intemperature, variations among the actuators and so forth, or to adapt toa plurality of image-quality modes.

Accordingly, the maximum voltage impressed to the actuator 1 is set to80V, and the maximum voltage impressed to the actuator 2 is set to 46V.Thereupon, since an optimal bias voltage impressed to the commonelectrode according to the present invention is approximately half thevoltage impressed to the actuator, the bias voltage impressed to thecommon electrode is 40V for the actuator 1, and the bias voltageimpressed to the common electrode is 23V for the actuator 2.

As above, since the maximum voltage capable of biasing is 54V for theactuator 1, the actuator 1 involves no problem. On the other hand, themaximum voltage capable of biasing is 23V for the actuator 2, which isequal to the bias voltage impressed to the common electrode. Inconsideration of reliability, in the actuator 2, the bias driving methodusing the direct-current voltage in which the voltage is constantlyimpressed cannot be selected. However, even in the actuator 2, if thevoltage impressed to the common electrode is a pulse-form voltage, theactuator 2 can be used.

Further, when the actuator is driven for a long time, adiabaticexpansion and contraction are repeated in the gap between the electrodesso as to induce moisture. This moisture causes a hydrogen bond and aliquid bridging force so as to increase an adsorption force between thecontacting electrodes. Therefore, even when the restitution force of thediaphragm is large, driving the actuator for a long time causes aphenomenon in which the diaphragm is stuck to the electrode, if thedirect-current bias voltage exists.

Therefore, impressing a pulse-form voltage to the common electrodeensures reliability of the drop discharge head.

Next, a description will be given of polarities of potentials impressedto the common electrode and the individual electrodes.

The above-mentioned level shifter shown in FIG. 11 is a positive voltageconversion level shifter that converts 5V into 12V, for example. On theother hand, the level shifter shown in FIG. 12 is a negative voltageconversion level shifter that converts 5V into −12V, for example. Ineach of the level shifters, a voltage input from an input terminal Vinis subjected to level-shifting, and is output in-phase from Vout2.

Here, a description will be given of the negative voltage conversionlevel shifter shown in FIG. 12. When a voltage VH is input to the inputterminal Vin, a P-channel MOSFET PMOS2 turns on so that the voltage VHis impressed to a drain of an N-channel MOSFET NMOS2. Besides, anN-channel MOSFET NMOS1 turns on by the voltage VH being impressed to agate thereof, and impresses a negative voltage VL to a gate of theN-channel MOSFET NMOS2.

Accordingly, as is well known, a withstand pressure required between thegate and the drain of the N-channel MOSFET NMOS2 becomes |VH|+|VL|. Thisapplies similarly to the N-channel MOSFET NMOS1.

That is, when using voltages having the same absolute value, transistorscomposing the negative voltage conversion level shifter require a largerwithstand pressure than transistors composing the positive voltageconversion level shifter.

Besides, in order to increase a reversing speed upon a signal reversal,the P-channel MOSFETs PMOS1 and PMOS2 of the negative voltage conversionlevel shifter may often need to have a gate width larger than that ofthe positive voltage conversion level shifter, depending on amanufacturing process to be used. However, since this method increasespower consumption during the course of reversal, there is another methodusing additional transistors for increasing the reversing speed.

As described above, when using voltages having the same absolute value,the negative voltage conversion level shifter becomes larger, i.e., morecostly, than the positive voltage conversion level shifter.

Accordingly, when driving an electrostatic head by the head drivecontrol device (adopting the bias method) according to the presentinvention by using the driving waveform as shown in FIG. 6E whichapplies positive potentials to numerous individual electrodes andapplies a negative potential to only one or several common electrodes,the drive control device can be made inexpensive and small.

Next, a description will be given, with reference to FIG. 14 and FIG.15, of an example of an inkjet recording device according to the presentinvention. FIG. 14 is a perspective view of a mechanism part of therecording device. FIG. 15 is a side sectional view of the recordingdevice.

In this inkjet recording device, a printing mechanism part 212 and soforth are contained in a recording device body 211. The printingmechanism part 212 comprises a carriage 223 movable in a main scanningdirection, recording heads 224 composed of the inkjet heads according tothe present invention mounted on the carriage 223, ink cartridges 225supplying inks to the recording heads 224, and so forth. A feedingcassette (or a feeding tray) 214 capable of carrying multiple sheets 213can be inserted detachably from a front side in a lower part of the body211. Besides, a manual feeding tray 215 can be opened for manuallyfeeding the sheets 213. The sheet 213 fed from the feeding cassette 214or from the manual feeding tray 215 is taken into the recording device,and a desired image is recorded by the printing mechanism part 212.Thereafter, the sheet 213 is delivered to a delivery tray 216 attachedat a rear side.

The printing mechanism part 212 holds the carriage 223 slidably by amain guide rod 221 and a sub guide rod 222 which are guide membersprovided horizontally across right and left side boards not shown in thefigures so that the carriage 223 is capable of sliding freely in themain scanning direction (a direction perpendicular to a surface of FIG.15). The heads 224 composed of the electrostatic inkjet headsdischarging ink drops of colors of yellow (Y), cyan (C), magenta (M) andblack (Bk) are mounted on the carriage 223 so that a plurality of inkdischarge openings are arranged in a direction crossing the mainscanning direction, and that a direction of discharging the ink dropsfaces downward. The ink cartridges 225 for supplying the inks of therespective colors to the heads 224 are also mounted replaceably on thecarriage 223.

Each of the ink cartridges 225 includes an air opening at an upper part,a supply opening at a lower part, and a porous member in an inner part.The air opening communicates with air. The supply opening supplies theink to the inkjet head. The porous member is filled with the ink inside.The ink supplied to the inkjet head is maintained at a slight negativepressure by a capillary force of the porous member.

Besides, although the heads 224 of the respective colors are used asrecording heads in the present embodiment, one head including nozzlesdischarging the ink drops of the respective colors may be used instead.

The main guide rod 221 is inserted into the carriage 223 slidably at therear side (downstream in a sheet conveyance direction of conveying thesheets). The carriage 223 is put on the sub guide rod 222 slidably atthe front side (upstream in the sheet conveyance direction). Besides, inorder to move the carriage 223 in the main scanning direction, a timingbelt 230 is stretched between a driving pulley 228 and a driven pulley229 rotated by a main scanning motor 227. The timing belt 230 is fixedto the carriage 223, and the carriage 223 is driven back and forth byforward and backward rotations of the main scanning motor 227.

On the other hand, in order to convey the sheet 213 set in the feedingcassette 214 to beneath the heads 224, a feeding roller 231, a frictionpad 232, a guide member 233, a conveying roller 234, a conveyance roller235, and a leading-edge roller 236 are provided. The feeding roller 231and the friction pad 232 separate and feed the sheet 213 from thefeeding cassette 214. The guide member 233 guides the sheet 213. Theconveying roller 234 reverses and conveys the fed sheet 213. Theconveyance roller 235 is pressed against a circumference of theconveying roller 234. The leading-edge roller 236 regulates an angle atwhich the sheet 213 is sent out from the conveying roller 234. Theconveying roller 234 is rotationally driven by a sub-scanning motor 237via a series of gears.

In addition, a print receptacle member 239 is provided. The printreceptacle member 239 is a sheet guide member guiding the sheet 213 sentout from the conveying roller 234 beneath the recording heads 224 inaccordance with a moving range of the carriage 223 in the main scanningdirection. A conveyance roller 241 and a spur 242 are provideddownstream from the print receptacle member 239 in the sheet conveyancedirection. The conveyance roller 241 and the spur 242 are rotationallydriven so as to send out the sheet 213 in a delivery direction. Further,a delivery roller 243, a spur 244, and guide members 245 and 246 areprovided. The delivery roller 243 and the spur 244 send out the sheet213 to the delivery tray 216. The guide members 245 and 246 form adelivery path.

Upon a recording operation, the recording heads 224 are driven accordingto an image signal while moving the carriage 223 so as to record oneline by discharging the inks to the halted sheet 213, and record a nextline after conveying the sheet 213 by a predetermined distance. Uponreceiving an end-of-recording signal or a signal indicating an arrivalof a trailing end of the sheet 213 at a recording area beneath therecording heads 224, the recording operation is ended and the sheet 213is delivered.

Besides, a recovery device 247 for recovering a discharge fault of theheads 224 is provided at a position rightward in the moving direction ofthe carriage 223 outside the recording area. The recovery device 247includes a capping part, a sucking part, and a cleaning part. During astandby for printing, the carriage 223 is moved to the recovery device247, and the heads 224 are capped by the capping part so as to keep thedischarge openings in a wet state, thereby preventing a discharge faultoriginating from a drying of the ink. Besides, inks irrelevant ofrecording are discharged during the recording, etc., so as to makeviscosity of the inks at all of the discharge openings constant, therebymaintaining a stable discharge performance.

In cases, such as that a discharge fault occurs, the discharge openings(the nozzles) of the heads 224 are sealed hermetically by the cappingpart, and air bubbles and so forth as well as the inks are sucked out ofthe discharge openings by the sucking part via tubes; inks, dusts and soforth adhering to surfaces of the discharge openings are removed by thecleaning part, thereby recovering the discharge fault. Besides, thesucked inks are ejected to a waste ink holder (not shown in the figures)provided in the lower part of the body 211, and are absorbed in an inkabsorber in the waste ink holder.

In the heretofore-described inkjet recording device, the inkjet headscomposing the recording heads 224 are driven by the head drive controldevice according to the present invention. Thus, the recording heads 224can be formed by the inkjet heads having a high degree of nozzleconcentration at a low cost, thereby obtaining the inexpensive inkjetrecording device capable of recording with a high image quality.

Besides, the above-described embodiment is an example in which thediaphragms form the common electrode, and the opposing electrodes as thesecond electrodes form the individual electrodes, since the diaphragmsare unitarily formed throughout the actuators so that the diaphragmsforming the first electrodes are combined electrically throughout theactuators. However, as mentioned above, the opposing electrodes as thesecond electrodes may be combined electrically throughout the actuatorsso as to form the common electrode, and the diaphragms may be separatedfor each of the actuators so as to form the individual electrodes.

Additionally, the common electrode may be formed by the first electrodesor the second electrodes combined electrically throughout all of theactuators, as mentioned above; alternatively, all of the actuators maybe divided into a plurality of blocks, and a plurality of commonelectrodes may be formed for the blocks (a number of the commonelectrodes being smaller than a total number of the actuators)

Further, the inkjet head is explained above as an example of the dropdischarge head driven and controlled by the head drive control deviceaccording to the present invention. However, the head drive controldevice according to the present invention is also applicable for drivingand controlling a drop discharge head discharging a drop of liquidsother than ink, such as a liquid resist used for patterning, or a geneanalysis sample.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority application No.2002-272383 filed on Sep. 19, 2002, the entire contents of which arehereby incorporated by reference.

1. A liquid-discharge head including an electrostatic actuator whichincludes a nozzle discharging a liquid drop, a discharge roomcommunicating with the nozzle, a diaphragm having a first electrodeforming a wall surface of the discharge room, and a second electrodeopposing the first electrode, the diaphragm being deformed by generatingan electrostatic force between the first electrode and the secondelectrode, wherein the liquid-discharge head comprises: a first separateelectrode provided in a vicinity of the first electrode which contacts aportion of the second electrode when the diaphragm is deformed, thefirst separate electrode being electrically isolated from the firstelectrode; and a second separate electrode provided in a vicinity of thesecond electrode which contacts a portion of the first electrode whenthe diaphragm is deformed, the second separate electrode beingelectrically isolated from the second electrode.
 2. The liquid-dischargehead according to claim 1, wherein the first separate electrode and thesecond separate electrode are electrically connected to each other. 3.The liquid-discharge head according to claim 2, wherein the firstseparate electrode and the second separate electrode are connected to aground terminal.
 4. A recording device which discharges a liquid dropfrom the liquid-discharge head according to claim 1, wherein theliquid-discharge head includes a plurality of electrostatic actuators,and the first electrodes of the electrostatic actuators are combinedelectrically, wherein the liquid-discharge head comprises a partapplying differently polarized potentials to the first electrode and thesecond electrode upon discharging a liquid drop.
 5. The recording deviceaccording to claim 4, wherein said part applies a positively polarizedpotential to the second electrode.
 6. The recording device according toclaim 4, wherein maximum values of the different polarized potentialsapplied to the first electrode and the second electrode havesubstantially equal absolute values.
 7. The recording device accordingto claim 4, wherein waveforms of the differently polarized potentialsapplied to the first electrode and the second electrode are pulsewaveforms.
 8. A recording device which discharges a liquid drop from theliquid-discharge head according to claim 1, wherein the liquid-dischargehead includes a plurality of electrostatic actuators, and the secondelectrodes of the electrostatic actuators are combined electrically,wherein the liquid-discharge head comprises a part applying differentlypolarized potentials to the first electrode and the second electrodeupon discharging a liquid drop.
 9. The recording device according toclaim 8, wherein said part applies a positively polarized potential tothe first electrode.
 10. An image forming device which records an imageby discharging a liquid drop, wherein the image forming device comprisesthe liquid-discharge head according to claim 1.